The invention relates to load-commutated inverters in general, and more particularly to a synchronous motor drive embodying such inverter.
In a load-commutated inverter (LCI), the load supplies the means of commutation of the SCR devices of the inverter. This is achieved provided there is a leading phase displacement angle for the inverter output current with respect to the load voltage. When the load is a synchronous motor, a leading power factor can be achieved. However, the need for running the motor with a leading current displacement angle and the resulting lower power factor affect adversely the efficiency of the motor drive, the motor and inverter being inefficiently utilized, a situation which is seriously worsened when the operation is performed at a high frequency.
The need for a leading power factor with the load lies in the need for natural commutation of the SCR's of the inverter. This problem has been explained in the following published articles:
"Utilization and Rating of Machine Commutated Inverter-Synchronous Motor Drive" by J. Rosa in IAS78-15A, pp. 453-457.
"A Self-Controlled Synchronous Motor Drive Using Terminal Voltage Sensing" by H. Le-Huy, A. Jakubowicz and P. Perret in IEEE 1986, pp. 562-569.
"System Design Method for a Load-Commutated Inverter-Synchronous Motor Drive" by Allan B. Plunkett and Fred O. Turnbull, Vol. IA-20, Trans. IEEE, May/June 1984.
Analysis of a Novel Forced-Commutation Starting Scheme for a Load-Commutated Synchronous Motor Drive" by R. L. Steigerwald and T. A. Lipo, IEEE Trans. Ind. Appl., Vol. 1A-15, pp. 14-24, January/February 1979.
While a leading load power factor ensures natural commutation in the quenching process and normally operates also during the thyristor recovery time, at high operative frequency the thyristor recovery time might prevent the total extinction of the outgoing static power switch in the inverter, if the margin angle .delta. is not large enough to exceed the value 2.pi.f.sub.m T.sub.q, where f.sub.m is the motor frequency and T.sub.q the thyristor recovery time. This is a particularly serious drawback with a variable speed AC motor drive.
An object of the present invention is to provide, in a load commutated inverter, assistance in the commutation of the outgoing SCR by generating an auxiliary voltage effective during the entire recovery phase.
This approach falls short of any forced-commutation arrangement of the prior art, since the leading power factor approach to natural commutation remains and is exercised during the quenching phase, thereby requiring only a limited and inexpensive solution to the commutation problem. In addition, while allowing a load commutated inverter to operate under a leading power factor, this is done without having to lower the power factor to the extent required when operating in the high frequency range, thereby preventing load utilization under a poor power factor.